Neoarchaean Ages Research Articles

Provenance of the Trainor's Rocks microconglomerate, Northern Ireland: a mid-Silurian (Hawick Group) submarine channel fan deposit in the closing Iapetus Ocean

A pebbly gritstone–microconglomerate outcrop at Trainor's Rocks in the Mourne Mountains is one of the youngest ( c . 430 Ma) coarse clastic units within the Ordovician–Silurian Southern Uplands–Down–Longford Terrane. The c . 400 m by 1 km outcrop lies within fine-grained Hawick Group (Wenlock) strata, and is rich in extraformational clasts including granite, rhyolite, andesite, basalt, vein quartz and metamorphic rocks, plus intraformational rip-up clasts of mudstone. Detrital zircons, analysed by sensitive high-resolution ion microprobe (SHRIMP), yield predominantly Ordovician U–Pb ages of 450–490 Ma, peaking at c . 470 Ma, coincident with arc-related magmatism in the Midland Valley Terrane. Whole-rock geochemical data are consistent with derivation from a calc-alkaline continental arc, with clast provenance matching 473–464 Ma arc volcanic and intrusive rocks from the Tyrone Igneous Complex. A small proportion of analysed zircons have Proterozoic and Neoarchean ages typical of sediments derived from the Dalradian Supergroup (Grampian Terrane). The youngest zircon analysed, 435 ± 8 Ma, may indicate that magmatism continued during closure of Iapetus in the Llandovery. In these samples there is no evidence for Gondwana-derived sediment. The influx of detritus from the volcanic arc–Laurentian hinterland suggests episodic tectonic unroofing in response to syndepositional strike-slip movements.

U–Pb age dating and geochemical fingerprints of Cambrian granites in the North Moroccan Meseta. Implication for the continental rift opening in the Northwest Gondwana margin

The Imouzzer Emsian conglomerate of the North Moroccan Meseta contains pebbles of granites. The granites are peraluminous and highly potassic, with a calc-alkaline trace element pattern (high lithophile element enrichment and Nb and Ta negative anomalies). They are dated to late Miaolingian (ca 500 Ma) by U–Pb zircon. The magmatic zircons have inherited cores of Ediacaran, Palaeoproterozoic, and Neoarchaean ages. Rb–Sr and Sm–Nd isotopic analyses display epsilon Nd values at 500 Ma ranging between −5.6 and −1.4. The TDM ages after DePaolo (1981) are calculated between 2701 and 1330 Ma. According to geochemical and isotopic data, the granitic magma may have originated from the melting of lower crustal Ediacaran to Palaeoproterozoic protoliths including Archaean remnants. This acidic magmatic occurrence is contemporaneous with neighbouring rhyolitic activities of bimodal volcanic complexes, which are related to the opening of the Cambrian intra-continental rift of Morocco.

Neoproterozoic granitoids and rhyolites of Wrangel Island: Geochemical affinity and geodynamic setting in the Eastern Arctic region

New results from the study of zircon U–Pb ages by SIMS SHRIMP and LA–ICP–MS, Nd isotopy and the geochemistry of granitic plutonism and felsic volcanism of Wrangel Island are considered. Granitic intrusions are confined to the Wrangel Complex in the Southern tectonic zone, cropping out in the core of a latitudinally extending anticlinorium and representing the metamorphic basement of the Eastern Arctic shelf. Felsic and basic volcanic rocks form a belt of scattered outcrops in the Central tectonic zone. The obtained U–Pb data form two Neoproterozoic age clusters in the Wrangel Complex at ~680–720 and ~590–600Ma corresponding to the times of granitic plutonism and felsic volcanism, respectively. Granites that are 680–720Ma old contain inherited zircons of Meso-, Palaeoproterozoic, and Neoarchaean ages, implying the existence of ancient rocks in the basement of Wrangel Island and their participation in partial melting during granite magma formation. The negative εNd(Т) (−2.97 to −4.46) values obtained for granites suggest the contribution of a crustal component to their petrogenesis, and their model ages indicate possible Palaeo-Mesoproterozoic ages of crustal protoliths. The geochemical affinities of granites are similar to those of highly fractionated peraluminous I-type granites, and their association with clastic and volcanic rocks of felsic, intermediate, and basic compositions provides grounds for assuming their formation on an active continental margin. Similar formation settings above subduction zones are also proposed for Neoproterozoic (600–700Ma) granites and orthogneisses of the Seward Peninsula (Amato et al., 2009, 2014). The close association of ~590–600-Ma-old felsic volcanic rocks with geochemical affinities of A-type granites and coeval enriched basalts (Moiseev et al., 2015) makes them similar to a bimodal rift-related assemblage. The moderate positive εNd(Т) (1.21; 1.1) values and Mesoproterozoic model T(DM) and T(DM-2) ages obtained for felsic volcanic rocks lead us to assume a heterogeneous source of magmas, which included both crustal and mafic mantle components. Similar rift-related geodynamic settings were proposed for orthogneisses (565Ma) and gabbro (540Ma) of the Seward Peninsula, Alaska (Amato et al., 2009, 2014) and orthogneisses (656 and 574Ma) of the Koolen gneiss dome, Chukotka, which indicate intense rifting. The rocks also exhibit some analogies with Neoproterozoic volcano-plutonic complexes of the Timanides grouped into I- and A-types (Kuznetsov et al., 2006; Pease et al., 2004). I-type granitoids are 700–515Ma old and represent constituents of the volcanic–plutonic series of an active continental margin. A-type granitoids have isotopic ages of 565–500Ma and are associated with volcanic rocks of the bimodal basalt–rhyolite series formed in extensional environments.

Study of Calc-Silicate Rocks of Hammer-Head Syncline from Southern Sandmata Complex, Northwestern India: Implications on Existence of an Archaean Protolith

Abstract Existence of an Archaean protolith is suggested in present study from an ensemble of rocks named as Sandmata Complex from northwestern India which have a debatable stratigraphic status of Archaean vs. Proterozoic. Rocks of the Sandmata Complex are represented by a highly metamorphosed volcano-sedimentary complex with multiple cycles of deformation. The manifold tectono-thermal events have obscured the pristine character of the protoliths. In this work we present geochemical features of calc-silicate protolith that show consistent Archaean affinity in the Hammer-Head Syncline (HHS) from southern part of the Sandmata Complex. Notable geochemical characteristics of calc-silicate metasediments in the HHS include high Th/U, high Cr concentrations, high La/Th, moderate La/Yb, and weak positive Eu anomaly. Carbon and oxygen stable isotope compositions of these carbonate metasediments vary between −3.0 and −0.3‰ (δ13Ccarb),-11.6 and −35.0 (δ13Corg) and −19.1 and −13.4‰, (δ18O) respectively. These geochemical observations are in conjunction with the recently published Neoarchaean ages from the HHS and the proximal Hooke syncline.

Salt domes of the UAE and Oman: Probing eastern Arabia

The emergent salt domes of the United Arab Emirates (UAE) have been investigated in detail and examples from central Oman have been studied for comparison. The salt domes contain exotic clasts of igneous, sedimentary and low-grade metamorphic rocks of the Arabian basement that have been brought to the surface from depths of over 8km. The clasts thus provide an opportunity to examine the lithology, geochemical composition and age of the “basement” underlying this part of eastern Arabia, where no other outcrops are available for direct study. Five volcanic rocks give consistent latest Ediacaran U-Pb zircon crystallisation dates of ca. 560–545Ma, with Neoproterozoic, Palaeoproterozoic and Neoarchaean ages of inherited zircons. These rocks, although strongly altered, preserve geochemical characteristics compatible with formation in a within-plate, extensional setting along the northern edge of Gondwana, adjacent to Prototethys. U-Pb analyses of detrital zircons in sedimentary and low-grade metamorphic rocks indicate deposition younger than ca. 597Ma in UAE and <734Ma in Oman. The two UAE sedimentary rocks may correlate with the Shuram and Khufai Formations of the Nafun Group (Huqf Supergroup) in Oman. Like the volcanic rocks, the two sedimentary samples from the UAE show derivation from the erosion of Neo-, Palaeoproterozoic and Neoarchaean sources. These sources could be from the Arabian basement itself or from other basement blocks such as those embedded in present-day Iran and Afghanistan, the precise whereabouts of which in Neoproterozoic times remains somewhat uncertain. The zircon age spectra of samples from the UAE show Neoproterozoic age peak characteristics of sources from both the western and eastern Arabian basement blocks, indicating that the two segments were juxtaposed by about 597Ma, the maximum age of their deposition.

Infrastructure and provenance of Ganderia: Evidence from detrital zircon ages in the Brookville terrane, southern New Brunswick, Canada

The Brookville terrane in southern New Brunswick contains some of the oldest known sedimentary rocks of the Ganderian microcontinent of the northern Appalachian orogen. It includes the stromatolitic metacarbonate- and quartzite-dominated Ashburn Formation and metasiltstone-dominated Martinon Formation, in mylonitic contact with the Brookville Gneiss, an assemblage of low-pressure/high-temperature paragneiss and tonalitic orthogneiss. All of the metasedimentary units were intruded by the ca. 550–528Ma subduction-related Golden Grove Plutonic Suite. Analysis by LA-ICP-MS and TIMS of zircon grains from the Ashburn Formation quartzite yielded no <10% discordant ages younger than ca. 1200Ma, a predominance of Meso- and Paleoproterozoic ages back to ca. 2100Ma, and a few Neoarchaean ages. A sample from the metasiltstone matrix of a carbonate olistostrome in the Martinon Formation contains rounded grains with ages mainly between ca. 1000Ma and 2200Ma, indicating provenance similar to that of the Ashburn Formation. However, the sample also contains euhedral grains with ages averaging around 650Ma, indicating the maximum depositional age. A quartzite/calc-silicate sample from the Brookville paragneiss yielded detrital zircon grains with most ages between 1150Ma and 2000Ma, indicating provenance similar to that of the Ashburn and Martinon formations. An orthogneiss sample yielded euhedral (igneous) zircon grains with a concordia age of 615±4Ma, indicating an interval of continental margin subduction at least 60Ma older than that represented by the Golden Grove Plutonic Suite. The Ashburn and Martinon formations and Brookville paragneiss are evidence for development of a Late Cryogenian passive margin on the Proto-Andean-Caribbean edge of Amazonia after it had separated from Laurentia by ca. 650Ma to open the Puncoviscana Ocean. By 615Ma at least part of this margin had become a subduction zone which remained active during the Late Neoproterozoic-Early Cambrian, part of the Pampean arc system. Brookville and related terranes rifted away in the mid-Cambrian to form the foundation of Early Paleozoic Ganderia, which ultimately collided with composite Laurentia during the Salinic orogeny.

Mid- to Late Cambrian docking of the Río de la Plata craton to southwestern Gondwana: age constraints from U–Pb SHRIMP detrital zircon ages from Sierras de Ambato and Velasco (Sierras Pampeanas, Argentina)

Abstract: The Early Palaeozoic stratigraphy and tectonic history of the Eastern Sierras Pampeanas of central Argentina are complicated by metamorphism and deformation resulting from the Pampean (545–510 Ma) and Famatinian (490–440 Ma) orogenies. We report U–Pb sensitive high-resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at Quebrada de La Cébila ( c . 28°45'S, 66°25'W): the Ambato and the La Cébila metamorphic complexes. The Ambato zircons record age peaks corresponding to Pampean (530 ± 10 Ma), Brasiliano ( c . 570 and c . 640 Ma), Grenville ( c . 950 to c . 1025 Ma) and minor Neoarchaean ages. Similar peaks are also apparent in the La Cébila sample but it additionally contains Palaeoproterozoic zircons ( c . 2.1 Ga) corresponding to the age of the Rio de la Plata craton, from which they are considered to have been sourced. Our interpretation is that the protolith of the Ambato complex was deposited prior to juxtaposition with the craton and is older than the Early Ordovician La Cébila metamorphic complex. We infer that the craton reached its current relative position in the Mid- to Late Cambrian, after the main Pampean tectonothermal event (530–520 Ma) and before deposition of the La Cébila protolith and the Achavil Formation (Sierra de Famatina), which contain comparable detrital zircon populations.

Detrital heavy minerals, white mica and zircon geochronology in the Ordovician South Mayo Trough, western Ireland: signatures of the Laurentian basement and the Grampian orogeny

Abstract: The Ordovician strata of the South Mayo Trough in western Ireland were deposited as a conformable sequence, first in the forearc of a north-facing oceanic arc, then in a synorogenic basin above the arc–continent collisional Grampian orogen, then in an extensional hanging-wall basin above the exhuming Dalradian metamorphic complex and, finally, in an Andean-type margin successor basin. This depositional history, and its relationship to the Grampian orogeny, to the pre- and post-collision arcs, and to sources in the Laurentian foreland, is recorded in detrital heavy mineral suites, Ar/Ar ages of detrital white micas, and U–Pb ages of detrital zircons. Heavy minerals record the progressive extensional unroofing of an obducted ophiolite complex, then the rapid extensional unroofing of the Grampian Dalradian metamorphic complex. White mica ages give Neoproterozoic, Mesoproterozoic, and Neoarchaean ages in the lower part of the sequence then, at about 466 Ma, synchronously with the appearance of amphibolite-facies metamorphic detritus, a tightly unimodal Grampian metamorphic age. Zircons yield mainly Mesoproterozoic and Neoarchaean ages, indicating a Laurentian source, and a Grampian age of 460–468 Ma. Paralleling the appearance of Grampian-age micas, the colour of zircon changes from dominantly purple to dominantly colourless, reflecting the thermal annealing effect of metamorphism. However, the temperature of Grampian metamorphism was not high enough to reset the U–Pb isotopic systematics of detrital zircons.